Tolylene diisocyanate, which is important for a raw material of polyurethane, has been industrially produced.
With a production plant for tolylene diisocyanate, tolylene diisocyanate is produced in the following manner. First, toluene as a raw material is dinitrated to produce dinitrotoluene, and the dinitrotoluene is subsequently reduced to produce tolylenediamine. Thereafter, the tolylenediamine is phosgenated to thereby produce tolylene diisocyanate.
However, in this production plant for tolylene diisocyanate, tolylene diisocyanate is produced to satisfy demand, and when the demand for tolylene diisocyanate is high, its production from toluene as the raw material may not be finished on time in some cases. In such case, it is effective to obtain tolylenediamine which is an intermediate raw material, and then produce tolylene diisocyanate from the tolylenediamine.
Such available tolylenediamine is prepared to have a water content of 5 to 15% by weight for storage and transportation (see, for example, the following Patent Document 1).
There has been proposed a method of dehydrating an aqueous solution containing about 60% by weight of diaminotoluene by distillation until it has a residual water content of 3.5% by weight, as a distillation method for separating amine from an amine aqueous solution (see, for example, the following Patent Document 2).
Further, there has been proposed a method for producing toluylene diisocyanate in the following manner. In a first production plant, first, toluene is used as a starting material to form dinitrotoluene, the dinitrotoluene thus formed is hydrogenated to thereby generate a crude solution of toluylene diamine and reaction water, the crude solution of toluylene diamine and reaction water is distilled to yield an intermediate mixture of toluylene diamine and water so that the water content in the intermediate mixture is about 1 to 40% by weight, and the intermediate mixture is transported to a second production plant.
Then, in the second production plant, the intermediate mixture is distilled to yield dry toluylene diamine, and followed by phosgenation of the toluylene diamine to give toluylene diisocyanate (see, for example, the following Patent Document 3).
In this production method, for example, the intermediate mixture of toluylene diamine and water is distilled in a first-stage distillation column to give toluylene diamine having a water content of about 7% by weight, and the toluylene diamine having a water content of about 7% by weight is distilled in a second-stage distillation column to thereby yield toluylene diamine having a water content of 400 ppm.    Patent Document 1: Japanese Unexamined Patent Publication No. 08-119910    Patent Document 2: Japanese Unexamined Patent Publication No. 62-212351    Patent Document 3: Japanese Unexamined Patent Publication No. 09-52873